1. Field of the Invention
The present invention relates to a photographic lens assembly, particularly to a photographic lens assembly of such type that a photographic lens system divided into at least two lens groups with an air space therebetween is provided and focusing of said lens system against an object is achieved by shifting said two lens groups along optical axis with such shift amounts as being different to each other.
2. Description of the Prior Art
The photographic lens for a camera is generally so designed that the best correction of aberrations can be achieved at the infinity setting, and as the subject distance decreases the variation of aberrations increases to impair the characteristics of the image. Thus displacement of the lens causes a corresponding change in the characteristics of the image. Photographic lenses of the retrofocus type extensively used for photographic cameras, when displaced, are subject to especially marked variations in aberrations. Moreover, with the tendency in recent years to shorten the closest subject distance, it has become an important problem to remedy the deterioration of characteristics of image due to the variation of aberrations.
To overcome this problem, it is known to divide the photographic lens system into a front lens group and a rear lens group and to move the two groups relative to each other so as to render the air space for infinity setting different from that for close-up setting and thereby correct the variation of aberrations when close-up are taken.
While this method has been proposed by U.S. Pat. No. 3,674,343 by the assignor in the present case granted to Ito et al. (filed Sept. 15, 1970, Ser. No. 72,364, "Lens Barrel," corresponding to German DAS 20 46 261), such arrangement assures good characteristics of image both for infinity setting and close-up setting.
Said "Lens Barrel" proposed by this U.S. Pat. No. 3,674,343 has:
a photographic lens system divided into a front lens group held by a first frame member and a rear lens group held by a second frame member with an air space therebetween which is variable by the displacement of the two lens groups relative to each other, PA1 an outer cylinder to be fixed to a camera body, PA1 a middle cylinder having two helical threads on both of the outer and the inner surface thereof and disposed in thread engagement with the outer cylinder through the outer thread, PA1 an inner cylinder disposed in thread engagement with the inner thread of the middle cylinder, and PA1 a focusing ring fixedly attached to the middle cylinder,
and is so made that a first key member fixedly attached to the outer cylinder is made to engage with key slot formed at a part of the inner cylinder for preventing the rotational action of said inner cylinder and restricting the same in the direction along the optical axis, at the same time the second frame member holding the rear lens group is fixedly attached to said inner cylinder, on the other hand a flange portion is formed at a forward end part of the first frame member holding the front lens group and said first frame member is functionally associated with the forward end part of the middle cylinder through said flange portion, at the same time a second key member fixedly attached to the second frame member is engaged with a key slot formed at a part of the first frame member for preventing rotational action of the first frame member and restricting the same in the direction along optical axis, so that by rotatingly operating the focusing ring the front lens group and the rear lens group are shifted linearly along optical axis based on the lead of outer thread of the middle cylinder and the synthesized lead of the outer and inner threads of the middle cylinder, respectively, that is with such shift amount as being different each other.
However, there are such disadvantages in said proposed "Lens Barrel" that the key member (that is the above mentioned second key member and the key slot) for regulating or guiding the first frame member to the direction along optical axis while preventing rotational action of the first frame member holding the front lens group is provided at the air space between the front and the rear lens group, thus said air space necessarily increases, therefore, the total length of the "Lens Barrel" increases and a compact "Lens Barrel" can not be obtained.
Next, secondly there are such disadvantages that both functional parts of the first frame member are positioned with too much gap therebetween in such manner that a first functional part (that is the above mentioned flange portion) to be coupled with the middle cylinder in being driven manner is positioned at a forward end part of said frame member while a second functional part (that is the above mentioned key slot) which is to be coupled with the second key member for preventing the rotational action of the first frame member and regulating or guiding the same to the direction along the optical axis, therefore, it is comparatively difficult to always make satisfactory linear driving of said first frame member, particularly when "Lens Barrel" is slanted in use the torque required for driving said first frame member largely varies thus smooth operation of the focusing ring is blocked.
Further, there are such disadvantages that a driving point for the first frame member and a driving point for the second frame member are positioned with too big gap therebetween in the focusing ring, therefore the driving of said both frame members will be very much unbalanced.